The present invention relates to a toner image fixing device that is used in electrophotographic copying machines, facsimiles, printers and the like machines using electrophotographic process.
In conventional electrophotographic copying machines, facsimiles, printers and the like machines using electrophotographic process, there is usually used such a toner image fixing device that fixes a toner image on a recording material passing through nip portion formed between a fixing roller and a pressure roller, by heating either one or both rollers (hereinafter called as pressure roller type). In the pressure-roller type fixing device, paired rollers must rotate in synchronism with each other and must be rotatably supported. Therefore, the device has a complicated construction and is large and expensive to manufacture.
To solve the above-mentioned problems, the present applicant proposed a toner image fixing method that fixes a toner image on recording paper transferring through nip portion formed between a fixing roller and non-rotating pressure member used instead of a pressure roller (Japanese Patent Application No. 7-44647). This method is hereinafter called as pressure pad type.
A pressure pad type toner image fixing device has a fixing roller and a pressure member disposed under the fixing roller. The fixing roller is comprised of a thin-wall aluminum cylinder whose external surface is coated all over with well-releasable, paper-guiding heat-resistant synthetic resin, e.g., heat-resistant silicone rubber having a large friction coefficient. Furthermore, the pressure and pad type toner image fixing device has a pressure member disposed under the fixing roller. A heat-resistant sheet on a lower frame which is interposed between the pressure member and the fixing roller. The heat-resistant sheet is made of a 100 micron thick glass fiber base coated or impregnated with well-releasable and heat-resistant synthetic resin, e.g., fluorocarbon resin: PFA (Tetrafluoroethylene-fluo-roalkylvinylether copolymer), PTFE (Polytetrafluoro-ethylene) and FET (Polytetrafluoroethylene-Polytetra-fluoropropylene copolymer). A recording paper with an unfixed toner image developed thereon passes nip portion formed between the fixing roller and the heat-resistant sheet for fixing the toner image thereon by fusing. The pressure-pad type fixing device involves such a problem that the fixing roller must have a toner-releasable property and a paper carrying property but could not yet satisfy both requirements because these two properties are contrary to each other.